2

Different causes:
If the pain doesn't go down your legs then it is unlikely to be a disc related problem. Other possibilities include facet joint arthritis, the beginning of spinal stenosis or arthritis due to other causes. If the pain gets worse, is associated with fever or chills, vision changes you need some intensive testing. If not consider some otc analgesics like aleve.
...Read more

4

Pregnancy test:
I am unaware of how regular your periods are, but pregnancy is always a likely answer. it all depends on your cycle and exposure to sex. Usually I would do the test 1-2 weeks after a missed period, but you can do it now if you feel that you are pregnant. The back pain can be treated with tylenol, ibuprofen, while you wait, but it is always best not to take anything if you think pregnancy
...Read more

7

Misinformed?:
She is not pregnant. Period. Having a period on time and expected means she did not get pregnant. Having sex just before menstruation is not risky for pregnancy . One gets pregnant at ovulation. Much earlier in the cycle. Her current symptoms do not in any way represent pregnancy.
...Read more

8

Muscular likely:
You describe a very common and annoying type of low back problem, one that affects millions of people in this country. Female patients, the younger patient should be seen and examined for rheumatologist conditions but on average this is muscular type straining, considering lack o urinary, bowel and midline spine pain. Gentle stretching, cool compresses and tylenol and or advil should help resolve.
...Read more